Assessment of Aircrew Head Injury Protection After Helmet Liner Modification
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USAARL-CNPA-BC--2022-07
UNITED STATES ARMY AEROMEDICAL RESEARCH LABORATORY
Assessment of Aircrew Head Injury
Protection After Helmet Liner Modification
Shelby N. Sous, Greg A. Ganz, Katie P. Logsdon, &
Frederick T. Brozoski
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1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From - To)
01-11-2021 Briefing Charts
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Assessment of Aircrew Head Injury Protection after
Helmet Liner Modification 5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
Sous, S. N.1,2, Ganz, G. A.1,3, Logsdon, K. P.1, & Brozoski, F. T.1 MOMRP17090
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORT NUMBER
U.S. Army Aeromedical Research Laboratory
P.O. Box 620577 USAARL-CNPA-BC--2022-07
Fort Rucker, AL 36362
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)
U.S. Army Medical Research and Development Command USAMRDC
MOMRP
810 Schreider Street 11. SPONSOR/MONITOR'S REPORT
Fort Detrick, MD 21702 NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
DISTRIBUTION STATEMENT A. Approved for public release; distribution unlimited.
13. SUPPLEMENTARY NOTES
1U.S.Army Aeromedical Research Laboratory; 2Katmai Health Services, Anchorage, AK; 3Oak Ridge Institute for Science and
Education
14. ABSTRACT
U.S. Army aviators experiencing difficulty attaining a comfortable helmet fit are referred to the U.S. Army Aeromedical Research
Laboratory’s ProblemFit (PROFIT) program. USAARL PROFIT personnel make modifications to the comfort liner to
accommodate atypical head anthropometry to achieve a more user-specific fit. Effects of these modifications on blunt impact
performance of the HGU-56/P Aircrew Integrated Helmet System (AIHS) and head injury protection are unknown. It is important
for these effects to be quantified and the risks associated with modifications to be understood by USAARL PROFIT personnel, the
aviator, and aviation leadership. Through a USAARL PROFIT records audit, two approved comfort liners were selected for
modification in the HGU-56/P AIHS: Super Comfort Liner™ (SCL) and Thermal Plastic Liner® (TPL). In order to evaluate a
“worst-case scenario” modification the two inner-most layers of thermoplastic material were removed from each liner. Helmets were
tested according to HGU-56/P AIHS Purchase Description (DoD, 1996) using a monorail drop tower (ANSI, 1966).
15. SUBJECT TERMS
HGU-56/P, AIHS, ThermoPlastic Liner, TPL®, Super Comfort Liner, SCL™, helmet problem fit, modified comfort liner,
blunt impact protection
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF Loraine St. Onge, PhD
PAGES
SAR 19b. TELEPHONE NUMBER (Include area code)
UNCLAS UNCLAS UNCLAS 22 334-255-6906
Standard Form 298 (Rev. 8/98)
Prescribed by ANSI Std. Z39.18
REPORT DOCUMENTATION PAGE (SF298)
(Continuation Sheet)
14. Abstract (continued)
The monorail drop tower was instrumented to collect three channels of data: impact velocity, headform acceleration, and impact force. Helmet
blunt impact performance was assessed using the peak headform acceleration for each impact. Two test series were conducted. Series one
consisted of six helmets each of the modified SCL™ and modified TPL® tested in the hot environmental condition. Series two consisted of six
helmets with the modified TPL® tested in the ambient condition. In series one, the modified SCL™ helmets tested in the hot environmental
condition peak headform accelerations averaged 112.51G at the crown and 141.63G at the headband. The modified TPL® helmets peak
headform accelerations averaged 112.69G at the crown and 144.71G at the headband. In series two, the modified TPL® helmets peak
headform accelerations averaged 124.16G at the crown and 148.65G at the headband. Results indicated the removal of the two inner-most
layers of the SCL™ or TPL® does not degrade the blunt impact protection of the helmet below the pass-fail threshold of 150G and 175G for
crown and headband impacts, respectively. In an isolated incident, a single helmet failed on rear impact. Helmet orientation and liner
installation are possible contributing factors.
Acknowledgements
This research was supported in part by an appointment to the Established Scientist
Program at the U.S. Army Aeromedical Research Laboratory administered by the Oak Ridge
Institute for Science and Education through an interagency agreement between the U.S.
Department of Energy and the U.S. Army Medical Research and Development Command.
This page is intentionally blank.
Assessment of Aircrew Head Injury Protection
after Helmet Liner Modifications
Shelby N. Sous1,2, Greg A. Ganz1,3, Katie P. Logsdon1, Frederick T. Brozoski1
1U.S.Army Aeromedical Research Laboratory, Fort Rucker, AL
2Katmai Government Solutions, LLC, Anchorage, AK
3Oak Ridge Institute for Science and Education, Oak Ridge, TN
USAARL UNCLASSIFIED 1
The views, opinions, and/or findings contained in this briefing are those of
the author(s) and should not be construed as an official Department of the
Army position, policy, or decision, unless so designated by other official
documentation. Citation of trade names in this presentation does not
constitute an official Department of the Army endorsement or approval of
the use of such commercial items.
USAARL UNCLASSIFIED 2
This research was supported in part by an appointment to the
Postgraduate Program at the U.S. Army Aeromedical Research Laboratory
administered by the Oak Ridge Institute for Science and Education through
an interagency agreement between the U.S. Department of Energy and the
U.S. Army Medical Research and Development Command.
USAARL UNCLASSIFIED 3A
1960s: Aircrew Protection Helmet Number 5 (APH-5)
• Hard shell aviation helmet
• Leather-covered foam pads
1969: Sound Protection Helmet Number 4 (SPH-4)
• Sling suspension system B C
1984: Integrated Helmet and Display Sighting System (IHADSS)
• Front and rear pads
• Inner basket assembly, vertically adjusted
• Brow and nape pads
D E
USAARL UNCLASSIFIED 4The Evolution of Helmet Fitting (Continued)
A
Mid-1980s: SPH-4 Conversion Kits
• Kit to modify to sling suspension
• Gentex ThermoPlastic Liner (TPL®) introduced
• Decrease headaches and hotspots
• Improve helmet stability
B
1989: SPH-4B
• Standard issue with TPL®
1995: Head Gear Unit Number 56 Personal (HGU-56/P) Aircrew
Integrated Helmet System (AIHS)
• Standard issue with TPL®
C
Alternative Liners
• Gentex Super Comfort Liner (SCL™)
• 2008: Oregon Aero Zeta II®
USAARL UNCLASSIFIED 5Helmet Fitting Issues
• Among aviators, individual anthropometric variability
incongruent with standard helmet fitting is the primary source
of discomfort.
• Extended duration of the “normal flight mission”
• Creates expected discomfort from head-borne mass
• Exacerbates pre-existing discomfort
• Subset of U.S. Army aviators continue to have problems
achieving a comfortable and safe helmet fit even with standard
adjustments or helmet liners.
USAARL UNCLASSIFIED 6USAARL PROFIT
• A safe and comfortable fit is considered mission critical in aviation.
• U.S. Army Aeromedical Research Laboratory (USAARL) Problem Fit (PROFIT) program
• Helmet fitting services for helmet fitting issues for the last 25+ years (Program
Executive Officer Aviation [PEO-AVN], 1996)
• If unit Aviation Life Support Equipment (ALSE) technicians cannot resolve issues with
basic modifications, aviators are referred to the USAARL PROFIT program for
evaluation.
• USAARL PROFIT technicians are authorized to make minor modifications to TPLs® and
SCLs™.
• Unique head anthropometry and specific complaint (e.g., hotspots, headaches) are
considered in PROFIT assessments for liner modification.
• Examples: Removing portions of one or multiple layers, removing a whole
layer
• There are limited data available to determine if these modification influence blunt impact
protection.
USAARL UNCLASSIFIED 7Study Objective
• Quantify the effects of helmet liner modifications on the
blunt impact protection provided by the HGU-56/P AIHS
• Comparing blunt impact attenuation testing results of
helmets with modified TPL® and SCL™ liners to
standard pass/fail criteria
USAARL UNCLASSIFIED 8Common PROFIT Modifications
• An audit of PROFIT records was conducted.
• Researchers identified the most common PROFIT modification
techniques used in the program:
• Removal of the two inner-most layers of thermoplastic material from
either the TPL® or the SCL™ was the most frequent modification.
• Other documented modifications removed portions of either the first or
second thermoplastic layers, or both.
• No modifications were documented where the two outer-most
thermoplastic layers were altered.
USAARL UNCLASSIFIED 9Liner Modifications
• A single helmet liner modification was selected for evaluation.
• Removed the two inner-most layers of the TPL® and SCL™ liners.
• This modification represented the most frequent and “worst-case” scenario.
• Custom fitting instructions from section 4-12 of the HGU-56/P AIHS
operator’s manual (Department of the Army [DA], 1996) were
modified:
• Modified TPLs® and SCLs™ liners were heated in a 200⁰ Fahrenheit (F) oven.
• Heating time was reduced from 10 minutes to 8 minutes based on the reduced
number of layers.
• Liners were installed in new HGU-56/P AIHSs.
• Helmets were donned on a spare headform and positioned upside down. The
weight of the headform simulated the downward pressure typically applied by
the wearer.
USAARL UNCLASSIFIED 10Test Methodology
• To maximize test assets, modifications were made to the
standard methodology.
• Each helmet was impacted in 5 of the 7 locations:
• Crown
• Headband regions (front, rear, left headband, and right
headband).
• Earcup impacts were omitted.
• The 5 impacts were separated into “For Record” and
“Supplemental.”
• “For Record impacts” – first 3 impacts to each helmet defined in the purchase
description (DOD, 1996)
• “Supplemental” – the additional 2 impacts to each helmet
USAARL UNCLASSIFIED 11Test Methodology
• Liner modification evaluations were divided into 2
series.
• Series 1: 12 size small HGU-56/P AIHSs were
tested after hot conditioning (122° F) for a minimum
of 4 hours. Results were used to down-select for series 2.
• Six helmets with modified TPL®
• Six helmets with modified SCL™
• Series 2: 6 size small HGU-56/P AIHSs were tested
in ambient conditions (60° to 80° F) with a modified
TPL® only.
USAARL UNCLASSIFIED 12Blunt Impact Attenuation Testing
• “For Record” impacts were completed according to the HGU-56/P
AIHS purchase description (Department of Defense [DoD],1996).
• A free fall monorail drop tower conforming to Federal Motor Vehicle
Safety Standard (FMVSS) 218 (Department of Transportation
[DoT], 2006) was used.
• Headband impacts (front, rear, left headband, and right headband )
were conducted at an impact velocity of 19.7 feet per second (fps).
• Crown impacts were conducted at an impact velocity of 16.0 fps.
• Impact was made on a flat steel anvil at the base of the drop tower.
• Headform acceleration, impact force, and impact velocity were
recorded for each drop test.
USAARL UNCLASSIFIED 13Data Analysis
• Headform acceleration data collected during each
blunt impact test were filtered according to Society
of Automotive Engineers (SAE) Standard Practice
J211-1 Part 1 (SAE, 1995).
• Peak headform accelerations were extracted from
each filtered acceleration signal.
• Blunt impact protection was assessed by comparing
the peak headform accelerations to the pass/fail
criteria specified in the HGU-56/P AIHS purchase
description (DoD, 1996).
USAARL UNCLASSIFIED 14Series 1: Small HGU-56/P AIHS with TPL® (hot conditioning)
⁰
USAARL UNCLASSIFIED 15Series 1: Small HGU-56/P AIHS with SCLTM (hot conditioning)
⁰
USAARL UNCLASSIFIED 16Data Analysis and Results
• Series 1 results were evaluated using a two-factor analysis of variance
(ANOVA) with the Tukey multiple comparison method for the effects of
helmet liner type and impact site on peak headform acceleration (p <
0.05).
• The two-way ANOVA tested for the equality of the mean peak headform
acceleration for each liner type and impact site.
• The ANOVA results were used to determine which modified liner would be
used for Series 2.
• Only the rear impact location showed a significant difference
between the mean peak headform accelerations measured with the
TPL® versus the SCL™ (p=0.006)
• The TPL® was chosen for ambient blunt impact evaluation for Series
2.
USAARL UNCLASSIFIED 17Series 2: Small HGU-56/P AIHS with TPL® (ambient conditioning) USAARL UNCLASSIFIED 18
Discussion
• Removing the two inner-most layers of thermoplastic material from TPLs® and SCLs™
did not adversely affect the impact protection provided by the HGU-56/P AIHS.
• HGU-56/P AIHS size small modified TPL® and SCL™ helmet liners were shown to meet
the blunt impact requirements outlined in the HGU-56/P AIHS purchase description
(DoD, 1996).
• Three rear impacts (one “For Record” and two “Supplemental”) on HGU-56/P
AIHSs with modified liners conditioned at 122° F resulted in a peak headform
acceleration above the specified pass-fail criterion (DoD, 1996).
• Results of all rear impacts indicate that the one “For Record” peak headform
acceleration exceeding 175 Gs may be an outlier, as other previously-damaged
(“supplemental” impacts) helmets fitted with modified TPLs® were shown to
provide the required level of impact protection.
USAARL UNCLASSIFIED 19Conclusions
• Per current fitting guidance, a four-layer TPL® or SCL™ is
required to be worn in the HGU-56/P AIHS (DA, 1996).
• Removing the two inner-most layers of thermoplastic material
from TPLs® and SCLs™ did not adversely affect the impact
protection provided by the HGU-56/P AIHS.
• Currently, such advanced liner removal or modifications should
only be made by trained USAARL PROFIT Program
technicians.
USAARL UNCLASSIFIED 20Questions
For details, see Brozoski, F. T., Ganz, G., Sous, S., Gomez, J., & Logsdon, K. (2020).
Assessment of HGU-56/P aircrew integrated helmet System (AIHS) blunt impact
protection after liner modifications (USAARL-TECH-FR--2020-018). U.S. Army
Aeromedical Research Laboratory.
USAARL UNCLASSIFIED 21U.S. Army Aeromedical Research Laboratory
Fort Rucker, Alabama
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